1. Field of the Invention
The present invention relates to a wafer grinder, and in particular to a wafer grinder that withholds lateral force and accurately makes adjustment to a worktable.
2. Description of Related Art
As is widely known in semiconductor industries, how to improve a total thickness variation (TTV) of a wafer grinder is a current topic of interest. The related technique includes a feedback circuit and an improved mechanism suitable for implementation in wafer grinders with high precision. Thus, it meets requirement of wafer grinding in the modern era. Utility and cost-cutting of the wafer grinder are widely recognized.
Generally, the wafer grinder includes an air pressure worktable and an air pressure spindle and is used to machine or mill brittle material. A vacuum disk is made of porous ceramic and is positioned above the air pressure worktable. A plurality of air holes are positioned within the air pressure worktable and used as the air pressure spindle. When milling or machining wafers with diamond wheels, the spindles of the diamond wheels are bent by a reaction force and are tilted. In addition, temperature of the diamond wheels rises so that the position of the diamond wheels is not in alignment with the wafers.
In the prior art, U.S. Pat. No. 5,567,199 discloses a conventional wafer grinder. Referring to FIG. 1, it illustrates a schematic drawing of a conventional wafer grinder. A displacement meter 31a is positioned on a worktable 22a and is used to measure displacement of a piezoelectric actuator 3a. Then, the displacement meter 31a transmits measurement data to a central computing unit (CPU) 32a, and the piezoelectric actuator 3a adjusts its displacement in response to measurement data of the displacement meter 31a. Thus, a wafer 2a is held by a vacuum disk 21a and rotated in a certain rotational direction. The wafer 2a is ground by a grinding wheel 1a due to relative movement between the wafer 2a and the grinding wheel 1a. However, the piezoelectric actuator 3a is subjected to a lateral force during the grinding process, which has a significant bad impact on the precision of grinding. The '199 patent discloses two features. Firstly, a feedback system is additionally positioned at the displacement meter 31a and is used to measure the displacement of the piezoelectric actuator 3a. However, a measuring device and an object measured are not in the same geometrical position, so the measurement is prone to error. Secondly, the piezoelectric actuator 3a is used to adjust the worktable 22a and is positioned below the worktable 22a. Because the piezoelectric actuator 3a is subject to the weight of the worktable 22a, the piezoelectric actuator 3a is prone to damage.
Additionally, U.S. Pat. No. 5,816,895 is shown in FIG. 2. The conventional wafer grinder utilizes four piezoelectric actuators 3b to adjust a tilt angle of a worktable 22b. The four piezoelectric actuators 3b are positioned below the worktable 22b and separated by 90 degrees. Alternately, four piezoelectric actuators 12b are positioned at a spindle of a wafer grinder 1b to adjust the tilt of the spindle of the wafer grinder 1b. Further, three displacement meters 31b are arranged over the wafer (not numbered) and transmit a thickness of the wafer to a central computing unit (CPU) 32b during the grinding process. A piezoelectric device 34b is used to control the piezoelectric actuators 12b and 3b so that the wafer 2b is ground by the wafer grinder 1b because the worktable 22b is adjusted by the piezoelectric device 34b. However, because the piezoelectric actuator 3a is subject to the weight of the worktable 22a, it has a significant impact on the precision of the grinding process. The '985 patent discloses two features. Firstly, a feedback system is additionally positioned with the displacement meters 31b and is used to measure the thickness of the wafer 2b. Thus, a measuring device and an object measured are not in the same geometrical position, so the measurement is prone to error. Secondly, the piezoelectric actuator 3b is used to adjust the worktable 22b and is positioned below the worktable 22b. It is thus prone to damage.
As described in U.S. Pat. Nos. 5,567,199 and 5,816,895, the adjustment mechanism of the worktable is positioned below the worktable to support the weight of the worktable and has no pre-compression device. As described in the '199 patent, a detection system is positioned near the spindle of the grinding wheel and is prone to contamination by machine oil. According to the '199 patent, the detection system of the piezoelectric actuator is a strain gauge attached to the surface of the piezoelectric actuator. Because the strain gauge and the piezoelectric actuator are in the same position, the displacement is regarded as an input value.
FIG. 3A illustrates how a wafer 2c is ground by a grinding wheel 1c. The wafer 2c is ground by friction between the wafer 2c and the grinding wheel 1c during the grinding process. Further referring to FIG. 3B, the predetermined position 12d of the grinding wheel 1c does not coincide with the practical position 11d of the grinding wheel 1c. Thus, it renders non-uniform the thickness of the wafer 2d. 
Thus, there is need to develop a mechanism for adjusting a machining angle of a wafer.